1. Field of the Invention
The present invention relates to a constant current control power supply and a laser oscillator, and more specifically relates to a constant current control power supply and a laser oscillator that sharpen rising and falling edges of an output current, while reducing a ripple current due to a disturbance variation by lowering a gain in a high frequency band in power control.
2. Description of Related Art
To drive laser diodes (LDs) used in laser oscillators, LD drive power supplies that output a constant current in accordance with a command value from a controller are necessary. The LD drive power supplies require high efficiency, a low ripple current, and a high speed response.
Regarding the requirement for high efficiency, SMPSs (switch mode power supplies), that is, so-called switching regulators are generally used to reduce a power loss (for example, Japanese Unexamined Patent Publication (Kokai) No. 2010-49523, which will be hereinafter referred to as Patent Literature 1). Contrary to this, configurations in which a constant voltage power supply and a constant current circuit are combined have worse conversion efficiency (for example, Japanese Patent No. 3256090).
A low ripple current is required of direct lasers (such as fiber lasers) because the direct lasers have a short time constant of electrical-to-optical conversion and slight current variations affect a laser output. As techniques for reducing a ripple current, there are generally known smoothing circuits having configurations in which a reactor and a capacitor are combined or added (for example, Patent Literature 1), instead of configurations having only a reactor (for example, Japanese Patent No. 3456121 and Japanese Unexamined Patent Publication (Kokai) No. 2003-234534, which are hereinafter referred to as Patent Literatures 3 and 4), to attenuate a fundamental ripple component at a switching frequency.
A high speed response is required of laser machining, which needs a pulse current of a high frequency (up to several tens of kHz), to enable (turn-on) and disable (turn-off) a laser at high speed. Thus, there are proposed a technique in which switches connected in parallel with LDs are turned on and off (for example, Patent Literatures 3 and 4), and a technique in which a switch is connected to LDs in series and an amplification factor (gain) of a difference (error amount) from a command value obtained by feedback is instantaneously raised upon turning on the switch and gradually lowered with time (for example, Patent Literature 1).
FIG. 1 is a block diagram of a conventional constant current switching power supply. The conventional constant current switching power supply, for controlling a current flowing through a load 1004, includes a switching circuit 1001, a rectifier circuit 1002, a smoothing circuit 1003, a switching device 1005 for the load, a current detection device 1006, an analog-to-digital converter (ADC) 1007, and a computing circuit 1008. The switching device 1005 switches on and off to enable and disable a current flow through the load 1004 in response to a pulse signal PS fed from a pulse generator 1009. To the computing circuit 1008, a current command Ic, which determines the value of the current flowing through the load 1004, is supplied by an external control circuit 1010.
As described above, the constant current switching power supply described in Patent Literature 1 is an appropriate example that satisfies all the requirements for high efficiency, a low ripple current, and a high speed response. However, in the case of switching on and off the switching device 1005 connected in series to the load 1004 such as LDs, as described in Patent Literature 1, there is a problem that it takes time to charge a capacitor of the smoothing circuit 1003 upon switching on. Patent Literature 1 discloses a technique for shortening the time to charge the capacitor upon switching on. However, the complicated control requires a processor such as a microcomputer or a DSP (digital signal processor). Moreover, since the processor also performs power control, a higher switching frequency requires a more expensive processor having higher computing power for a stable operation.